1. Field of the Invention
This invention relates to a GaP pure green light emitting element substrate, and more precisely to a GaP pure green light emitting element substrate comprising a plurality of GaP layers formed on a GaP single crystal substrate, which is used to manufacture high-brightness GaP pure green light emitting elements.
2. The Prior Art
Light emitting elements such as light emitting diodes are normally obtained by layering a plurality of semiconductor layers on a semiconductor substrate to prepare a multi-layer semiconductor substrate with a pn junction, and then making it into elements for practical use. Among them, green light emitting diodes can be obtained by using a light emitting element substrate prepared by forming one or more layers of both n-type and p-type GaP, one after another, on an n-type GaP single crystal substrate.
GaP is an indirect transition-type semiconductor, and therefore the brightness is very low if a pn junction is just formed. Because of this, nitrogen (N), which would function as light emitting centers, is added to the n-type GaP layer near the pn Junction in order to enhance the brightness. However, a light emitting diode prepared from a GaP light emitting element substrate which has the nitrogen-added n-type GaP layer emits a yellowish-green light with a peak wavelength of about 567 nm and therefore it does not give a pure green light with a peak wavelength of about 555 nm.
Because of this, a structure with a lowered carrier concentration in the n-type GaP layer or p-type GaP layer near the pn junction has been proposed for the purpose of otaining pure green light emission with a high brightnes without adding nitrogen.
FIG. 5 shows a cross-sectional structure of a GaP pure green light emitting diode which has an n-type GaP layer with a low carrier concentration (net donor concentration) formed on an n-type GaP layer with a high concentration of the impurity (electrodes are not shown). This type of cross-sectional structure is disclosed in Japanese unexamined patent publication (Tokkai) Sho 59-214276, for example. In this GaP pure green light emitting diode, as shown in FIG. 5 (A), an n-type GaP layer 51, a low-carrier-concentration n-type GaP layer 52 and a p-type GaP layer 53 are formed, one after another, on an n-type GaP single crystal substrate 50, and said low-carrier-concentration n-type GaP layer 52 functions as the light emitting layer.
In this GaP pure green light emitting diode, as shown in FIG. 5 (B), the net donor concentration (n) in the low-carrier-concentration n-type GaP layer 52 is set between 5.times.10.sup.15 to 1.times.10.sup.16 atoms/cm.sup.3, substantially lower than that in the p-type GaP layer 53. This improves the injection efficiency of the holes from the p-type GaP layer 53 into the low-carrier-concentration n-type GaP layer 52, thus improving the light emitting efficiency.
FIG. 6 shows a structure (electrodes are not shown) of a GaP pure green light emitting diode, which is disclosed in Japanese unexamined patent publication (Tokkai) Sho 57-93589. This GaP pure green light emitting diode has a p-type GaP layer with a lower carrier concentration (p) compared to the carrier concentration (n) in the n-type GaP layer which constitutes one side of the pn junction (said p-type GaP layer being a p-type GaP layer to which impurity as shallow donors has been added so that the net acceptor concentration is lowerd). In this GaP pure green light emitting diode, an n-type GaP layer 61, a low-carrier-concentration p-type GaP layer 62 which has a carrier concentration lower than that in the n-type GaP layer 61, and a p-type GaP layer 63 are formed on an n-type GaP single crystal substrate 60, and said low-carrier-concentration p-type GaP layer 62 functions as the light emitting layer.
In the low-carrier-concentration p-type GaP layer 62 (doped with Zn) of this GaP pure green light emitting diode, the carrier concentration (N.sub.A (acceptor concentration )-N.sub.D (donor concentration)) is kept low by adding shallow donors (impurities) such as S (sulfur) in the range of the concentration N.sub.D, 1.times.10.sup.16 .ltoreq.N.sub.D .ltoreq.3.times.10.sup.17 atoms/cm.sup.3, and the shallow donors in the low-carrier-concentration p-type GaP layer 62 become light emitting centers.
However, even with the conventional GaP pure green light emitting diodes as described above, pure green light emission with a enough high brightness could not be obtained, and so GaP pure green light emitting diodes, which would give pure green light emission with even higher brightness, have been desired.